Fuel cells are used to produce electricity when supplied with fuels containing hydrogen and an oxidant such as air. A typical fuel cell includes an ion conductive electrolyte layer sandwiched between an anode layer and a cathode layer. There are several different types of fuel cells known in the art; amongst these are solid oxide fuel cells (SOFC). SOFC are regarded as highly efficient electrical power generator that produces high power density with fuel flexibility.
In a typical SOFC, air is passed over the surface of the cathode layer and a fuel containing hydrogen is passed over the surface of the anode layer opposite that of the cathode layer. Oxygen ions from the air migrate from the cathode layer through the dense electrolyte to the anode layer in which it reacts with the hydrogen and CO in the fuel, forming water and CO2 and thereby creating an electrical potential between the anode layer and the cathode layer of about 1 volt.
Each individual SOFC is mounted within a metal frame, referred to in the art as a retainer, to form a cell-retainer frame assembly. The individual cell-retainer frame assembly is then joined to a metal separator plate, also known in the art as an interconnector plate, to form a fuel cell cassette. The cassettes are stacked in series with a seal disposed between the sealing surfaces of each cassette to form a SOFC stack.
Seals for SOFC stacks require special properties such as a coefficient of thermal expansion comparable to those of the components of the SOFC stacks, a suitable viscosity to fill the any gaps in the sealing surfaces of the cassettes, ability to maintain a hermetic seal at operating temperatures of 500° C.-1000° C., good chemical stability, and long term sustainability.
For high temperature operations of a SOFC stack in the 800° C. to 1000° C. range, an alkaline earth aluminosilicate glass, such as a barium-calcium-aluminosilicate based glass, also known as G-18 glass, developed by Pacific Northwest National Laboratory (PNNL), is utilized for SOFC stack sealing applications. G-18 glass provides a seal material that offers high electrical resistively, high coefficient of thermal expansion, high glass transition temperature, and good chemical stability. However, G-18 glass crystallizes at prolonged elevated operating temperatures becoming hard and brittle over time, thereby losing its ability to fill in gaps in the sealing surfaces and its ability to provide a hermetic seal.
There is a need to have a seal material for a SOFC stack that provides the desirable properties of G-18 glass, but yet remains compliant at prolonged elevated operating temperature. There is still a further need to have a seal material that is self-healing to repair any weak points that may develop over time.